<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,initial-scale=1.0,user-scalable=no">
<title>测试</title>
</head>
<body>
<style>
* {
	padding: 0;
	margin: 0;
	box-sizing: border-box;
}
body {
	height: 100vh;
}
.top {
	height: 100px;
	background-color: brown;
}
.bottom {
	position: absolute;
	top: 0;
	left: 0;
	right: 0;
	bottom: 0;
	background-color: cadetblue;
}
.top + .bottom {
	top: 100px;
}
</style>	
<div class="top"></div>
<div class="bottom"></div>
</body>
</html>